envd: streaming RPCs starve behind reverse proxies that buffer responses

[staryxchen]

Summary

envd's server-streaming RPCs (Process.Start, Process.Connect,
Filesystem.WatchDir) don't advertise to reverse proxies that their
responses are streaming. Behind a stock nginx (default
proxy_buffering on), chunks are held until the proxy buffer fills,
which makes interactive PTYs appear to accept input but produce no
output until something forces a flush.

The usual workaround — globally disabling proxy_buffering — also
gives up caching and weakens proxy_next_upstream retries for unary
RPCs that don't need streaming. Letting envd signal intent per
response is more surgical.

Affected handlers

• process.Process/Start (process/start.go)
• process.Process/Connect (process/connect.go)
• filesystem.Filesystem/WatchDir (filesystem/watch.go)

Proposed fix

A small connect-go interceptor that sets X-Accel-Buffering: no on
server-streaming responses, wired into the existing
connect.WithInterceptors(...) chain. Unary and client-streaming RPCs
pass through unchanged.

X-Accel-Buffering is read by nginx (and other proxies that follow the
convention) on a per-response basis:

• https://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_buffering
• https://www.nginx.com/resources/wiki/start/topics/examples/x-accel/

I have a patch ready (~60 LoC in a new internal/services/streaming
package + 2-line wiring per service + unit tests). Happy to open a PR
if the direction is right.

[linear-code]

ENG-4074